Since chlorinated rubbers obtained by chlorinating polyisoprene rubbers such as natural rubber and synthetic polyisoprene are excellent in acid resistance, alkali resistance, chemical resistance, flame resistance, insulating properties, and the like, and have film-forming properties, they are used in varnishes, paints, vehicles for printing inks, films for packaging, adhesives, etc. In particular, highly chlorinated rubbers having a chlorine content (concentration of chlorine) of at least 55 wt. % are widely used as raw materials for anticorrosive paints excellent in drying characteristics.
There has heretofore been known, as processes for chlorinating polyisoprene rubbers, (1) a process in which a polyisoprene rubber is dissolved in a chlorinated hydrocarbon solvent, and chlorine gas is blown into the solution to react them, (2) a process in which a polyisoprene rubber is dispersed or emulsified in water before its chlorination, etc.
The process making use of a chlorinated hydrocarbon such as carbon tetrachloride or monochlorobenzene as a solvent for the chlorination is generally used because it permits even chlorination of polyisoprene rubbers and can give highly chlorinated rubbers soluble in organic solvents. However, the use of the chlorinated hydrocarbons shows a tendency to be limited from the viewpoint of global environmental protection in recent years. Therefore, it has been increasingly desired to develop a chlorination process in an aqueous medium instead.
On the other hand, according to the conventional process in which a polyisoprene rubber is dispersed or emulsified in water before its chlorination, even chlorination is difficult. In addition, rubber particles tend to aggregate in the course of the chlorination, and there is a problem that the resultant chlorinated rubber is hard to be dissolved in a usual solvent such as toluene.
With the foregoing circumstances in view, the present inventors carried out a study of a process for evenly chlorinating a polyisoprene rubber in an aqueous medium without causing aggregation of particles to prepare a highly chlorinated rubber so soluble in organic solvents that it can be used as a raw material for heavy-duty anticorrosive paints. As a result, the present inventors previously proposed a process in which a synthetic rubber or natural rubber is added to an acid aqueous solution to prepare a dispersion of fine particles of the rubber, and the dispersion is then subjected to chlorination to a chlorine concentration of 55-75 wt. % in the presence of a surface active agent (Japanese Patent Application Laid-Open No. 46905/1992).
In this process, the chlorination is conducted by adding a dispersion or emulsion of the synthetic rubber or natural rubber to the acid aqueous solution and blowing chlorine gas into the dispersion or emulsion. The acidity of the aqueous solution is as relatively low as pH of about 1-7. In each of Examples thereof, an aqueous solution containing hydrogen chloride in a concentration of 1-2 wt. % is used. This process has excellent features compared with the conventionally-known process making use of an aqueous medium. However, it involves problems that when the polyisoprene rubber is chlorinated under conditions making use of the acid solution within a pH range of about 1-7, it is difficult to fully prevent the aggregation of particles, the resultant chlorinated rubber has insufficient solubility in organic solvents such as toluene, resulting in the formation of insolubles in some cases, and moreover difficulties are encountered upon sufficiently increasing the solution viscosity of the chlorinated rubber when dissolved in toluene.